BUILD: makefile: do not restrict Lua's prepend path to empty LUA_LIB_NAME

The HLUA_PREPEND_PATH and HLUA_PREPEND_CPATH settings were only applied
when LUA_LIB_NAME was empty, otherwise they were silently ignored. Let's
take them out of that conditional block as this makes no sense to enforce
such a restriction (the main reason in fact is that this whole block is
unreadable).

Also take this opportunity to unfold the last two imbricated tests of
LUA_LIB_NAME and put comments around certain blocks to know what "endif"
matches what "if".

Makefile

@@ -604,25 +604,29 @@ check_lua_lib = $(shell echo "int main(){}" | $(CC) -o /dev/null -x c - $(2) -l$
 LUA_LIB       =
 LUA_LD_FLAGS := -Wl,$(if $(EXPORT_SYMBOL),$(EXPORT_SYMBOL),--export-dynamic) $(if $(LUA_LIB),-L$(LUA_LIB))
 
+# Try to automatically detect the Lua library if not set
 ifeq ($(LUA_LIB_NAME),)
-# Try to automatically detect the Lua library
 LUA_LIB_NAME := $(firstword $(foreach lib,lua5.4 lua54 lua5.3 lua53 lua,$(call check_lua_lib,$(lib),$(LUA_LD_FLAGS))))
+endif
+
+# Lua lib name must be set now (forced/detected above)
 ifeq ($(LUA_LIB_NAME),)
 $(error unable to automatically detect the Lua library name, you can enforce its name with LUA_LIB_NAME=<name> (where <name> can be lua5.4, lua54, lua, ...))
 endif
+
 ifneq ($(HLUA_PREPEND_PATH),)
 OPTIONS_CFLAGS  += -DHLUA_PREPEND_PATH=$(HLUA_PREPEND_PATH)
 BUILD_OPTIONS += HLUA_PREPEND_PATH=$(HLUA_PREPEND_PATH)
-endif
+endif # HLUA_PREPEND_PATH
+
 ifneq ($(HLUA_PREPEND_CPATH),)
 OPTIONS_CFLAGS  += -DHLUA_PREPEND_CPATH=$(HLUA_PREPEND_CPATH)
 BUILD_OPTIONS += HLUA_PREPEND_CPATH=$(HLUA_PREPEND_CPATH)
-endif
-endif
+endif # HLUA_PREPEND_CPATH
 
 OPTIONS_LDFLAGS += $(LUA_LD_FLAGS) -l$(LUA_LIB_NAME) -lm
 OPTIONS_OBJS    += src/hlua.o src/hlua_fcn.o
-endif
+endif # USE_LUA
 
 ifneq ($(USE_PROMEX),)
 OPTIONS_OBJS    += addons/promex/service-prometheus.o